A gas flow pressure regulator is a device that reduces a relatively high input pressure to an adjustable, relatively constant lower outlet pressure. FIG. 1 depicts a conventional pressure regulator 100. The pressure regulator 100 may include an upper or range assembly 10 and a lower assembly 20, and a diaphragm 30 positioned between the upper and lower assemblies. The range assembly 10 is fairly conventional and may include a range spring 12 and associated support and housing structures as are known in the art.
The lower assembly 20 includes an inlet 22, an outlet 24, and a valve assembly 40 for regulating the pressure of a gas flow from the inlet to the outlet. The valve assembly 40 may include a valve spring 42 for acting upon a valve 44. The valve 44 includes a valve poppet 46 contiguous with a valve stem 47 that presses against the diaphragm 30. The valve 44 is biased by the valve spring 42 against a valve seat 48 as part of controlling the gas flow. The lower assembly 20 may include additional associated support structures as are known in the art.
The pressure regulator operates as follows to control the pressure of a flowing gas from the inlet 22, through the valve assembly 40, to the outlet 24. The valve spring 42 loads the valve 44 against the valve seat 48 to stop the flow of gas. For example, in an exemplary regulator used in semiconductor manufacturing, the valve spring may load the valve 44 against the valve seat with approximately three pounds of force to achieve a leak-tight seal in the absence of a range spring load. An upper end 14 of the range spring 12 is compressed by means of a threaded stem 16 to create a downward load. This drives the diaphragm 30 down to drive the valve 40 away from the valve seat 48, which allows gas to flow into the chamber 49 below the diaphragm 30. The diaphragm 30 typically is a resilient member with a relatively planar surface that is in communication with the valve stem 47. The diaphragm acts to prevent gas from escaping to the atmosphere, while being flexible enough to transmit the load from the range spring 12 to the valve assembly 40. A knob 11 in the upper assembly 10 may be turned by a user to apply a force to the range spring 12. This force of the range spring is transmitted by the diaphragm 30 to the valve assembly 40 to move the valve 44 from the seat 48 to permit the flow of gas through the valve. When the pressure acting on the diaphragm area generates a force equal to the range spring force, the system is balanced and the device will maintain that pressure to provide a constant pressure gas flow to the outlet.
FIG. 2 depicts a closer view of the pressure regulator 100 in the portion containing the range spring 12, diaphragm 30, and valve assembly 40. The legend of FIG. 2 identifies the pressure and force relationships by which this conventional pressure regulator operates, as are understood by those skilled in the art. As a result of such relationships, a relatively high pressure input gas flow may be converted to a constant and lower output gas flow across the regulator valve.
The force balance relationships depicted in FIG. 2 result in basic performance characteristics that can be represented as a curve. An exemplary performance characteristics curve is shown in FIG. 3, which shows the outlet pressure as a function of flow. As can be understood from FIG. 3, as the flow increases, the valve must open further resulting in a lower range spring force pushing down on the diaphragm. The result in turn is lower outlet pressure.
As the flow approaches zero, i.e., the valve approaches closure, the curve of FIG. 3 shows a sharp increase in the upward slope. This sharp slope increase is indicative of the force required to seal the valve 44 against the valve seat 48. In a flowing condition, the forces on either side of the diaphragm 30 are balanced in a manner such that the valve is positioned spaced apart from the valve seat to permit gas flow through the valve. As gas flow is reduced toward termination of the flow (i.e., the valve is being closed), the outlet pressure will rise to reduce the downward load on the valve until there is sufficient force between the valve and the valve seat to create a seal. The increase in pressure that occurs to produce the seal between the valve and valve seat is referred to in the art as “creep”.
Conventional pressure regulators as described above have deficiencies, which are depicted in FIGS. 4 and 5. As previously described, the force balance that ultimately determines the regulator outlet pressure is developed through the use of springs. The ideal regulator would have all components perfectly centered relative to one another, and all of the spring loads centered along the axis of the poppet travel. In actual performance, the components typically are not perfectly centered, and the force from the springs is not perfectly along the axis of poppet travel.
Conventional helical springs, such as those depicted in FIGS. 1 and 2, that are typically used in the construction of pressure regulators are fabricated with ground ends which lie in a plane that are from one to two degrees of perpendicular to the direction of poppet travel. In addition, the ends become even more non-perpendicular as the springs are compressed. The result of component misalignment and non-axial spring loads is a lateral load of the poppet against the seat. This non-axial, asymmetric spring load and the resultant lateral load are shown in FIG. 4. The lateral load of the valve against the valve seat increases the friction associated with the movement of the valve causing the valve to resist motion. The force for opening the valve, therefore, must initially overcome the increased frictional force before the valve can move.
By virtue of the increased frictional force, the valve tends to stick initially as the forces attempt to open the valve. When the opening force becomes sufficient to overcome the friction, the valve overshoots its ideal position, resulting in a temporary spike in the flow. The valve then recovers to its ideal position. This overshoot and recovery is shown on the chart of FIG. 5. The sudden change in pressure associated with the overshoot may undermine the performance of downstream components, particularly in relatively low flow systems. For example, downstream performance particularly may be undermined in mass flow controllers that are employed in the manufacture of semiconductor devices.
Valve seat wear is a second deficiency that arises from the misalignment of the components and forces in a conventional pressure regulator valve assembly. This also is of particular concern in valves having high valve stroke rates associated with their flow rates, which tends to cause far more seat wear than low stroke applications. The results of seat wear include an increase in the creep pressure and, eventually, leak across the seat. Degradation in performance due to high stroke cycling can be observed in as little as 2000 cycles.